Accidental spills, leaking underground storage tanks, improper disposal, and landfill leachates are often the sources of chlorinated aliphatic hydrocarbons (CAHs) in groundwater. Trichloroethylene (TCE) is the most frequently found contaminant in sites of the National Priority List, and cis-1, 2-dichloroethylene (c-DCE) and vinyl chloride (VC) are commonly present due to the transformation of higher chlorinated parent compounds. Aerobic co-metabolism is a potential biological process for treatment of CAHs both in situ and ex-situ. Here microorganisms stimulated on a primary electron donor fortuitously degrade the contaminant. The process involves relatively non-specific oxygenase enzymes that subsequently oxygenase enzymes that subsequently result in a one-step transformation of the CAHs. These oxygenase enzymes typically have low half-velocity co-efficients, and, as a result, achieve in low contaminant concentrations upon remediation. Co-metabolic degradation of CAHs, however, is known to produce toxic intermediate compounds. This resulting toxicity is an important process in the selection of the co-metabolite process because of its effect on the degradation kinetics. In addition, these toxic intermediate compounds are of concern because of their potential health of ecological impacts if they remain in the treated aquifer. In the proposed research we will expand the understanding of these toxic metabolic intermediates in relation to their identify, their impact upon CAH degradation. Studies and the mechanisms by which toxicity is exhibited. We will focus on the co- metabolism of VC, 1,1-DE, and c-DCE since less we know less about transformation toxicity for these compounds, compared to TCE, will be performed with pure bacterial cultures grown on ammonia, toluene, and butane, to access how co-metabolic transformation of the selected CAHs will be determined. The resulting toxicity will be assessed based on loss in growth substrate utilization activity. Also oxidized products, such as alcohols and organic acids, and oxygen consumption. The mechanism(s) of co-metabolic toxicity will be evaluated in Task 2 using 14C radiolabeled CAHs coupled with protein assay methods. Products of the co-metabolism will be tracked using 14C measurement methods. The distribution of the CAH intermediates will be determined by protein binding assays using the 14C-labeled CAHs. The effective of active substrate metabolism upon the degree of co-metabolic activity and the ability of cell to recovery from toxic effects will be evaluated in Task 3. In Task 4 the impact of o-metabolic toxicity on the composition of consortium in a mixed culture constructed of three different pure cultures, growing on a single substrate (touene and butane) will be evaluated. The changes in the community will be tracked using molecular methods and protein assay methods.